C++ Standard Library

In the C++ programming language, the C++ Standard Library is a collection of classes and functions, which are written in the core language and part of the C++ ISO Standard itself.[1] The C++ Standard Library provides several generic containers, functions to utilize and manipulate these containers, function objects, generic strings and streams (including interactive and file I/O), support for some language features, and everyday functions for tasks such as finding the square root of a number. The C++ Standard Library also incorporates 18 headers of the ISO C90C standard library ending with ".h", but their use is deprecated.[2] No other headers in the C++ Standard Library end in ".h". Features of the C++ Standard Library are declared within the stdnamespace.

A noteworthy feature of the C++ Standard Library is that it not only specifies the syntax and semantics of generic algorithms, but also places requirements on their performance.[5] These performance requirements often correspond to a well-known algorithm, which is expected but not required to be used. In most cases this requires linear time O(n) or linearithmic time O(n log n), but in some cases higher bounds are allowed, such as quasilinear time O(n log2n) for stable sort (to allow in-place merge sort). Previously sorting was only required to take O(n log n) on average, allowing the use of quicksort, which is fast in practice but has poor worst-case performance, but introsort was introduced to allow both fast average performance and optimal worst-case complexity, and as of C++11, sorting is guaranteed to be at worst linearithmic. In other cases requirements remain laxer, such as selection, which is only required to be linear on average (as in quicksort),[6] not requiring worst-case linear as in introselect.

The C++ Standard Library underwent ISO standardization as part of the C++ ISO Standardization effort, and is undergoing further work[7] regarding standardization of expanded functionality.

Components that C++ programs may use to perform seminumerical operations.

<complex>

The header <complex> defines a class template, and numerous functions for representing and manipulating complex numbers.

<random>

Facility for generating (pseudo-)random numbers

<valarray>

Defines five class templates (valarray, slice_array, gslice_array, mask_array, and indirect_array), two classes (slice and gslice), and a series of related function templates for representing and manipulating arrays of values.

Each header from the C Standard Library is included in the C++ Standard Library under a different name, generated by removing the .h, and adding a 'c' at the start; for example, 'time.h' becomes 'ctime'. The only difference between these headers and the traditional C Standard Library headers is that where possible the functions should be placed into the std:: namespace. In ISO C, functions in the standard library are allowed to be implemented by macros, which is not allowed by ISO C++.